A variety of polyether polyols are widely used as soft, flexible segments in the production of elastomeric block copolymers such as polyurethanes and polyether esters. In general, such polyether polyols are low to medium molecular weight polymers having low glass transition temperatures and at least two hydroxyl groups per polymer chain. The low glass transition temperature provides high elasticity and good low temperature performance, while the hydroxyl groups permit the polyether polyol to react with the other components of the segmented elastomers. Examples of commercially important polyether polyols include polyethylene glycol, polypropylene glycol, ethylene oxide/propylene oxide copolyols, and polytetramethylene ether glycol (poly THF).
It is desirable that a polyether polyol used as a soft segment have primary hydroxyl groups to provide good reactivity towards the electrophilic functional groups such as isocyanate or carboxylate present on the hard segment components. Furthermore, to develop optimum low temperature properties it is generally preferred that the polyether polyol be amorphous and not crystallizable. In addition, the polyether polyol should be hydrophobic since the mechanical properties of the segmented elastomer product can be adversely affected by absorption of water. The commonly used polyether polyols generally are either hydrophilic (polyethylene glycol and ethylene oxide/propylene oxide copolyols) or crystallizable (polytetramethylene ether glycol), or have secondary hydroxyl end-groups (polypropylene glycol).
For these reasons oxetane polyols and oxetane/oxolane copolyols have been investigated as polyether polyols of potential commercial interest since materials which are simultaneously amorphous, hydrophobic, and which have primary hydroxyl end-groups can be obtained by the selection of appropriate monomers. However, until now only a limited number of synthetic methods for the preparation of these oxetane-containing polyols have been developed.
Conjeevaram et al. (J. Polym. Sci., Polym. Chem. Ed., 23 (1985) 429) teach the preparation of polyoxytrimethylene glycol by either of two routes. In the first method, high molecular weight polyoxetane is synthesized using an aluminum coordination catalyst then ozonized and reduced with lithium aluminum hydride. In the second route, the polyoxytrimethylene glycol is obtained directly by the cationic polymerization of oxetane using boron trifluoride/ethyl ether as catalyst and a diol as co-initiator.
Toga et al. (U.S. Pat. No. 4,599,460) teach a process for producing a polyether polyol in which 3-methyloxetane and tetrahydrofuran are copolymerized at low temperature using a hydroacid catalyst such as perchloric or fluorosulfonic acid.
Motoi et al. (U.S. Pat. No. 4,672,141) teach preparation of a 3-methyloxetane polyol using a hydroacid catalyst to Polymerize the oxetane monomer at cryogenic temperatures.
All of these known methods for producing oxetane-containing polyols involve either a tedious, indirect route or the use of very low temperatures (&lt;-40.degree. C. Such methods are not practical or economical to carry out on a commercial scale. In addition, the known methods for forming the polyether polyol directly all employ a strong acid catalyst which is either expensive, difficult to handle, or highly toxic. Most of these strong acid catalysts are soluble in the polymerization mixture and thus difficult to remove and recycle in subsequent polymerizations.